Oily bores, volume II

[Reckless Rat]

Not wishing to intrude on the former thread, my RP has developed a slight problem where it oils/cokes up the plug on n°3 cylinder. In an attempt to reduce the problem I have changed over from B6HS plugs to a set of hotter running BP5HS. The car is a standard RP saloon with a 37 head and H1 1 1/8" SU carb.

On Sunday 4th we took the car out for the annual winter rally and I had to stop twice to change the plug on n°3 because the car started misfiring. The car also seemed to be a bit down on power, but I put that down to the fact that I had a slightly over average sized co-pilot... 

I have since done a compression test, thinking that it was a bore wear problem - the engine has been +.030 oversize since time mint imperial and had a new set of rings, valves & valve guides circa 2005, possibly a bit earlier.

The results of the compression test are as follows: (cylinders 1, 2, 3 & 4)

Cold:  (psi)  100, 100, 110, 95  (I put the slightly higher reading on n°3 down to the presence of oil in the bore).

Hot:   (psi)   130, 125, 125, 130

So I guess there's not much wrong there.

I will be removing the engine/gearbox when the weather gets a bit warmer because the clutch release bearing needs replacing, at which time I will strip the valves out, regrind and check the seats & valve guides. I suspect that oil is being drawn up the valve stem on n°3 cylinder - when I did the "Col de la Bonnette" some years back there was a valve sticking afterwards but I can't remember which.

Oh yes, one other thing. As I was stripping down the engine prior to the compression test it looked like the piston ion the carb was stuck in the down position, but I can't be sure if that was just down to me working on the engine or whether it had been like that during the run. Mind you plugs 1,2 and 4 were all the right colour. Just n°3 was acting up.

Apparently we're in "La Vie de l'Auto" again too...

Any other ideas?

[Greig Smith]

Possible broken ring ??

Depends on how hard you have been winding up the clockwork spring while climbing all those cols ??

Either way it should all reveal itself upon a tear-down

Cheers
Greig

[AllAlloyCup]

Worn inlet valve guide on the affected cylinder? Oil coming up a stud thread?
If the compression pressures are equal it’s hard to see how it’s a ring problem?

Regards

Bill G

Ps recently read a thread that said to use ptfe tape to seal head studs?

[Reckless Rat]

Im wondering whether it is a valve guide problem. Hopefully all will be revealed when I take it apart.

[Bob Culver]

Do Sevens ever draw significant oil through the guides? I would have thought there was little available.

In engines not severely worn or revved and whcih do not ping, broken rings are rare. Comression rings can have a major effect on oil consumption but if gaps reasonable one broken ring will be hard to detect from compression. Oil rings can break but usually by assemblers!
When SUs stick down you do not go anywhere!

[Charles P]

Do Sevens ever draw significant oil through the guides? I would have thought there was little available.

In engines not severely worn or revved and whcih do not ping, broken rings are rare. Comression rings can have a major effect on oil consumption but if gaps reasonable one broken ring will be hard to detect from compression. Oil rings can break but usually by assemblers!
When SUs stick down you do not go anywhere!

I'd look to the HT lead next before taking anything apart. A weaker spark can allow oiling.
Swap the HT lead to no 3 with no 4 and see if the problem moves.

Charles

[Reckless Rat]

50% of running round here is uphill, with the engine working hard, the remainder is downhill which is where the worn valve guide problem could manifest itself on over-run; although to be fair I haven't noticed any significant blue smoking when re-applying power. I take note of Charles' comment regarding the HT lead and I will swap them over to see if that helps or changes the location of the problem.